1. Field of the Invention
Aluminum is produced in Hall-Heroult cells by the electrolysis of alumina in molten cryolite using conductive carbon electrodes. During the reaction the carbon anode is consumed at the rate of approximately 450 kg/mT of aluminum produced under the overall reaction ##EQU1##
The problems caused by the consumption of the anode carbon are related to the cost of the anode consumed in the reaction above and to the impurities introduced to the melt from the carbon source. The petroleum cokes used in the anodes generally have significant quantities of impurities, principally sulfur, silicon, vanadium, titanium, iron and nickel. Sulfur is oxidized to its oxides causing particularly troublesome workplace and environmental pollution. The metals, particularly vanadium, are undesirable as contaminants in the aluminum metal produced. Removal of excess quantities of the impurities requires extra and costly steps when high purity aluminum is to be produced.
If no carbon is consumed in the reduction the overall reaction would be 2Al.sub.2 O.sub.3 .fwdarw.4Al+3O.sub.2 and the oxygen produced could theoretically be recovered. More importantly, when no carbon is consumed, there is no contamination of the atmosphere or the aluminum product from the impurities present in the coke.
Attempts have been made in the past to use non-consumable electrodes with little apparent success. Metals either melt at the temperature of operation, or are attacked by oxygen or by the cryolite bath. Ceramic compounds such as oxides with perovskite and spinel crystal structures usually have too high electrical resistance or are attacked by the cryolite bath. Electrodes consisting of metals coated with ceramics using conventional methods have also shown poor performance, in that almost inevitably, even the smallest crack leads to attack on the metal substrate by the molten salt bath.
2. Description of the Prior Art
Recently, efforts have been made to fabricate non-consumable electrodes from special compositions known as cermets. A cermet composition is defined as one consisting of both metallic and ceramic phases. The conventional method of preparing cermet compositions is to mix metal and ceramic powders, cold press a preform, and sinter the preform at an elevated temperature in a controlled atmosphere. Alternatively, the cermet may be prepared by hot pressing or hot isostatic pressing wherein the pressing and sintering operations are performed concomitantly. Cermets have high electrical conductivity in comparison to ceramic compositions and good corrosion resistance when compared to metals.
U.S. Pat. No. 4,374,050 to Ray provides a non-consumable electrode for molten salt electrolysis fabricated from at least two metals or metal compounds combined to provide a combination metal compound containing at least one of the group consisting of oxide, fluoride, nitride, sulfide, carbide or boride, the combination metal compound defined by the formula: ##EQU2## where Z is a number in the range of 1.0 to 2.2; K is a number in the range of 2.0 to 4.4; M.sub.i is at least one metal having a valence of 1, 2, 3, 4 or 5 and is the same metal or metals wherever M.sub.i is used in the composition; M.sub.j is a metal having a valence of 2, 3 or 4; X.sub.r is at least one of the elements from the group consisting of O, F, N, S, C and B; m, p and n are the number components which comprise M.sub.i, M.sub.j and X.sub.r ; F.sub.M.sbsb.i, F'.sub.M.sbsb.j, F'.sub.M.sbsb.i or F.sub.x.sbsb.r are the mole fractions of M.sub.i, M.sub.j and X.sub.r and 0&lt;.SIGMA.F'.sub.M.sbsb.i &lt;1.
U.S. Pat. No. 4,374,761--Ray relates to non-consumable electrodes for molten salt electrolysis comprised of a ceramic oxide composition and at least one metal powder dispersed through the ceramic oxide composition for purposes of increasing its conductivity, the metal powder selected from the group consisting of Ni, Cu, Co, Pt, Rh, In and Ir.
U.S. Pat. No. 4,397,729 to Duruz et al. discloses a non-consumable anode for molten salt electrolysis consisting of a cermet material formed from a ceramic oxide of, e.g., a ferrite or chromite, and a metal, e.g., a noble metal or alloy thereof.
European patent application No. 30,834 to Wheeler et al. discloses a non-consumable anode used in the production of aluminium from a cryolite-based fused bath containing alumina consisting of a sintered self-sustaining ceramic oxide body of spinel structure which is made conductive by selective partial substitution, the introduction of non-stoichiometry, or by doping so as to maintain the impurities in the produced aluminium at low levels. Preferred materials are partially-substituted nickel ferrite spinels.
U.K. patent application No. 2,069,529A to Duruz et al. provides a non-consumable anode for molten salt electrolysis consisting of a cermet material comprising at least one ceramic oxide such as chromite or ferrite of iron or nickel or ferric or chromic oxide and at least one metal such as nickel or chromium or a noble metal, e.g., palladium, or an alloy of such metals.